Who are the Chechens?They're an ethnic group in the Caucasus region that has been fighting for independence from Russia for four centuries. Most Chechens are Sunni Muslims, many of whom practice in the mystical Sufi tradition; most Russians are Orthodox Christians. But the independence struggle has long been more about nationality than religion. Chechen identity is fiercely independent and anti-hierarchical, and rests on clan honor and — to a great extent — hatred of Russia. Dzhokhar Tsarnaev's high school friends say he rarely talked about his ethnicity, except to quickly correct anyone who suggested he was Russian.

What do Chechens have against Russia?They deeply resent its attempts to dominate their homeland, which date to the 16th-century reign of Ivan the Terrible. In the Soviet era, the Chechens balked at the government's efforts to seize private property and collectivize farming and herding. Furious at their resistance, Stalin used false allegations that Chechens collaborated with Nazis as an excuse to deport almost the entire Chechen population — about 500,000 people — to the desolate steppes of Siberia and Central Asia during World War II. The transfer was one of the most devastating acts of ethnic cleansing in the 20th century; at least a third and possibly half of the Chechens died in the first year. Most of the survivors eventually returned to Chechnya, but the Tsarnaev family stayed through the 1990s in Kyrgyzstan, where Anzor, the father of Tamerlan and Dzhokhar, worked as a lawyer. While the father clearly inculcated a Chechen identity in his sons, any hopes he entertained of taking his growing family back to Chechnya for good evaporated when Chechens were forced to flee again.

What were they fleeing?Two brutal wars with Russia that together turned Chechnya into "probably the most dangerous heart of darkness in the world," in the words of historian Brian Glyn Williams. After the Soviet Union dissolved in 1991, Russian President Boris Yeltsin, carried away by his success in freeing Russia from the Soviet yoke, told all the Russian provinces to claim "as much autonomy as you can handle." Chechnya promptly declared independence, which was more than Yeltsin had had in mind. In 1994 he sent in troops to quickly reassert Russian sovereignty, but when Chechens fought back, the crackdown turned into a nightmarish slaughter. At least 50,000 civilians were killed; the capital, Grozny, was completely flattened; and some 200,000 women and children fled to neighboring provinces and farther afield. In 1999, then Prime Minister Vladimir Putin, who was preparing to succeed Yeltsin, blamed a series of deadly bombings in Moscow on Chechen militants and launched a second war.

What happened to Chechnya?It became a breeding ground for radicalism. So did the neighboring province of Dagestan, where the Tsarnaev family settled for a few years before coming to the U.S. about a decade ago. A few Arab jihadists came to help the Chechens fight and introduced Wahhabism, the very strict form of Islam dominant in Saudi Arabia. In 2002, Chechen Islamist militants seized a Moscow theater, and 41 of them died, along with 129 hostages, when Russian special forces moved in. In 2004, Chechen militants led a siege on a school in Beslan; more than 330 people, mostly children, were killed. And in 2010, two female Chechen suicide bombers killed more than 40 people in the Moscow subway. Now some Chechen militants may be looking beyond Russia for targets. Over the past two years, Chechens have been charged with involvement in terror plots in Spain and France, which were foiled. "If there is any connection between these kids and the insurgency there, it will be the first time they have struck a target outside of Russia," Georgetown University professor Christopher Swift told The Wall Street Journal. Chechnya itself has been ruthlessly pacified, but the militants are active in Dagestan, where Tamerlan stayed for six months, until last July.

What did Tamerlan do in the region?That is a major focus of the ongoing investigation. Even before he left on that trip, he'd grown markedly more religious and angry, his family says. Russian security services say he had already visited Dagestan in 2011, and was communicating with radicals there. In the last year, Dagestan has seethed with violence. Dozens of police were killed in operations against Islamist militants, a cleric was assassinated by bomb, and two suicide bombers hit the capital, Makhachkala, where Tamerlan was living with his father for some of the time. Tamerlan traveled twice to Chechnya as well, though only to visit cousins, his father claims. But upon his return to the U.S., Tamerlan made a YouTube playlist of Chechen protest songs and jihadist videos. One video is dedicated to the prophecy of the Black Banners of Khorasan, which says an invincible Islamic army will rise in Central Asia. "It is essentially an end-time prophecy," says Aaron Zelin, a fellow at the Washington Institute for Near East Policy. "This is definitely important in al Qaida's ideology." It's still far from clear, though, whether Chechnya's conflict-filled history and the radical ideas spawned there played a major role in the Boston Marathon bombings.

Chechens in the U.S.Very few — perhaps 1,000 — of the tens of thousands of Chechen refugees who fled the grim plight of their homeland have settled in the U.S. Most of them, like the Tsarnaevs, made a case for being refugees fleeing political or religious persecution at home. The small community is horrified by the actions attributed to two of their sons. "Most of us would be dead right now if it wasn't for the United States giving us a home and saving us from all the violence," said Boston resident Ali Tepsurkaev, who came to the U.S. about a decade ago. "It feels embarrassing for us,'' he said. "After all this hospitality we're getting from Americans, to hear that some Chechen... It's hard. It's difficult to explain."